Telecommunication systems can transfer telecommunication data such as voice calls, video, music, multimedia, text messages, email messages, multimedia messages, facsimiles and/or data. Telecommunication systems transfer telecommunication data between terminals over a telecommunications network. A terminal is a device that can transmit and/or receive telecommunication data. A terminal may be a fixed-line device or a mobile device. Telecommunication data may be transmitted over a telecommunications network using circuit-switching technology or packet-switching technology.
A telephony device is a telecommunication terminal that is suitable for transmitting and receiving voice calls over a telephone network. The telephony device may be a fixed-line telephony device that has a fixed location and communicates over a fixed-line telephone network. The telephony device may be a mobile telephony device that is portable and communicates over a wireless telephone network. The telephony device may transmit and receive the voice call over a circuit-switched telephone network or a packet-switched telephone network.
A Voice over Internet Protocol (VoIP) device is an example of a telephony device that communicates over an Internet Protocol (IP) network, such as the internet. In addition to voice calls, a VoIP device may provide short messaging, video conferencing, facsimile, voice mail, multimedia messaging and/or email. Although a number of different signalling protocols are available, a telephony device communicating over a packet-switched network typically uses Session Initiation Protocol (SIP) as a signalling protocol.
Subscribers have long had to deal with a proliferation of terminals through which third parties may contact them. For example, a business user might have multiple telephony devices including a fixed-line telephone for use in the office and a mobile telephone for use at other locations. Each telephony device has associated advantages; whilst a mobile telephone provides the user with mobility, a fixed-line telephone typically provides less costly call services, no need to recharge a battery, and also better service in areas where there is poor signal strength such as inside a building.
To avoid confusing calling parties with telephone dialling numbers for each of the telephony devices of a subscriber, a one-telephone dialling number telephony service allows a subscriber to publish a single telephone dialling number on which they can be contacted. Thus, when the single telephone dialling number is called, all of the subscriber's telephony devices will ring, for example simultaneously. The subscriber is then able to answer the call at the telephony device of his choosing.
One known attempt to provide one-telephone dialling number telephony services requires each physical telephony device to be assigned a unique telephone number. Hence, when a subscriber's published telephone dialling number is called, the terminating telephony system providing the one-telephone dialling number service will ring each device by establishing separate call legs using each device's unique telephone number. This type of one-telephone dialling number service is commonly referred to as a “SimRing” service.
Another known attempt to provide one-telephone dialling number telephony services can be found where all the telephony devices communicate over a packet-switched network using SIP. Each SIP-compliant telephony device registers contact information, such as IP address, against a common ‘address of record’ which typically maps to the subscriber's telephone number. When the subscriber's number is called, the terminating telephony system sends (or ‘forks’) the call to each telephony device using standard procedures found in SIP.
A further known attempt to provide one-telephone dialling number telephony services can be found where each telephony device is supported by a different telecommunications network. In this case, the one-telephone dialling number service uses multiple mechanisms to direct the calls over the different networks. For example, the one-telephone dialling number service may use SIP procedures to fork a call to a SIP compliant telephony device over a packet-switched network and use SimRing procedures to establish a call leg to a mobile telephone over a conventional wireless circuit-switched network using the mobile telephone's unique telephone number.
Following recent developments, telecommunication devices can now be multi-network devices that are configured to communicate with different types of network. For example, the latest generation of mobile telephony devices can now communicate over a packet-switched network in addition to the traditional circuit-switched network. This is achieved by using a “soft client” (a software programme) to establish calls over the packet-switched network. The soft client typically establishes calls over a packet-switched network using SIP as the signalling protocol.
It may be preferable for telephony data to be transferred over the packet-switched network rather than the circuit-switched network. Hence, when the soft client registers against the subscriber's address of record, the subscriber's one-number service will transfer any calls to the mobile telephony device over the packet-switched network, as well as to any other telephony device the subscriber has registered.
However, there may be occasions when the soft client is unreachable (for example, when the mobile telephony device has left the coverage of the WiFi hot spot). Nevertheless, since the mobile telephony device is a multi-network device, the circuit-switch network is usually still available for the transmission of calls.
In principle, a SimRing service could be used simultaneously to ring the mobile telephony device via a circuit-switched network just in case the soft client is unreachable. However, a consequence of this is that whenever the soft client is reachable the mobile telephony device receives the same incoming call via two completely independent paths. This can cause a number of undesirable problems, particularly when the soft client is not tightly integrated with the circuit-switched software. For example:                a conflict can occur between the soft client and the circuit-switched software as they both try to alert the subscriber of an incoming call. Depending on the mobile telephony device, this could result in a confusing user experience, or perhaps one will always “win” (for example, the circuit-switched software may take precedence, denying the user the opportunity to answer the call over the packet-switched network).        differences in the signalling delay paths may mean that the call is received over one path several seconds before it is received over the other path. During this time, the user may have already answered or chosen to ignore the call (for example, by silencing the ringing). However, when the call arrives via the other path, the mobile telephony device starts ringing again.        
An “unavailable call forwarding” service is known which could be used instead of the SimRing service—the unavailable call forwarding service could be set up to forward an incoming call to a mobile telephony device when the subscriber doesn't have any telephony devices which are currently reachable. However, the unavailable call forwarding service has limited application because it would fail to ring the mobile telephony device if the subscriber has any other reachable telephony devices registered to the service.
Embodiments of the present invention seek to address or overcome at least some of the above-mentioned problems associated with conventional telecommunication services.